1. Field of the Invention
The present invention relates to a vehicle height adjustment system for adjusting the vehicle height of a vehicle by supplying/discharging air to/from an air spring.
2. Description of the Related Art
Many large vehicles such as a heavy-duty truck have so-called air suspensions which use air springs as vehicle chassis springs.
Trucks or other large vehicles can secure a given suspension stroke irrespective of the weight of loaded cargos, by keeping the height of a body frame (vehicle height) at a predetermined height from an axle. Accordingly, it is possible to improve the ride comfort and also to reduce the vibration of a cargo bed effectively. The predetermined height is called a reference vehicle height and the value thereof is suitably determined depending on the vehicle model.
In particular, most of the trucks and the like having the air suspension system include a vehicle height adjustment system capable of adjusting the vehicle height by supplying and discharging air to/from the air spring. Specifically, this vehicle height adjustment system adjusts the vehicle height to keep it at a same level in the following manner, for example: a current vehicle height is obtained by detecting a distance between the axle and the body frame; the current vehicle height is compared with a predetermined reference vehicle height; and air is supplied from an air tank to the air spring or discharged from the air spring to the atmosphere, if the current vehicle height is different from the reference vehicle height. Moreover, among the conventional vehicle height adjustment systems, there is the one which obtains the average of vehicle heights measured during a predetermined period of time, and adjusts the vehicle height in accordance with the difference between the average vehicle height and the reference vehicle height, thereby preventing unnecessary consumption of air in the air tank. Here, the vehicle height is adjusted, for example, every time the body swings (see Patent Document 1, for example).
(Patent Document 1) Japanese Patent Laid-Open No. Hei 8(1996)-108731
Such conventional vehicle height adjustment system, however, has its own problem. When the air tank is empty or there is no sufficient air left therein, for example, the conventional vehicle height adjustment system first supplies air to the air tank and measures the internal pressure thereof with a pressure sensor. Then, after the internal pressure of the air tank has become a sufficient value, vehicle height adjustment control is started. That is, the conventional vehicle height adjustment system cannot start the vehicle height adjustment until the internal pressure of the air tank becomes a sufficient value.